[Click] CPU scheduling -> make elements interruptable.

[Eddie Kohler]

Hi Norbert,

Click is cooperatively scheduled, and despite that comment (which I just 
removed), there are no plans to change this.  There are no "quanta", and 
an element that enters an infinite loop will hang the whole router. 
This is probably how it will stay.

I STRONGLY discourage you from making elements preemptible.  This should 
be undertaken only if you understand Click's core deeply.  It can be 
done, of course, but would require serious changes to Click's task and 
RouterThread locking strategy.

However, you can still provide some semblance of fair CPU scheduling by 
preventing infinite loops in all elements, and penalizing long-running 
chains by running them much less often.  In the long run it will all 
work out, although it won't be "real time".  But do you really need real 
time?

Eddie



Egi, Norbert wrote:
> Hi,
>  
> We are using Click for desinging a shared forwarding engine with separate forwarding paths (chain of elements) per customer. The customers share all the NICs, so after the packets are polled out from the NIC's queue a Classifier demultiplexes them by emitting them to the proper forwarding path. Our intention is to provide a fair CPU scheduling for each of the forwarding paths. The stride scheduling algorithm, implemented in Click, would be a perfect solution for our problem, but after checking the source code and the available documents I found out that the algorithm hasn't been implemented completely as it was proposed. If I understand it correctly from the source and its comments, there is no such thing like "quantums" (i.e. a discrete time slice when the scheduled task is entitled to use the CPU) and I guess that's the main reason while the operation of the elements can not be interrupted.
>  
> In the first comment of lib/task.cc it's mentioned that this may be addressed in the future, so I was wondering whether anyone is working on it and I may jump in and help or just could someone provide information on how to make elements interruptable the fastest way extending the current version of the code.
>  
> Regards,
> Norbert
> 
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